Good: I've have a couple of the Polaris under seat bags. The standard under seat bag ($40 #2876427) is compact and fits right under the seat. The next larger bag is called the 121" under seat bag ($80 #Item #2878736) but fits the RMK's and has much more room. The problem I've had with both of these bags is that everything gets damp/wet inside.
Great: If I was going to get a new bag it would definitely be a Sled Solutions E Series Tunnel Bag $140. It is made way better, has more room, and is more versatile. It's kind of pricey, but you could always use it on a Pro if you decide to get a new sled in the future.